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Two pressures are measured for a blood pressure reading 1  : Systolic blood pressure is a measure of blood pressure while the heart is beating. 2  Diastolic pressure is a measure of blood pressure while the heart is relaxed.
Systolic: The blood pressure when the heart is contracting. It is specifically the maximum arterial pressure during contraction of the left ventricle of the heart. The time at which ventricular contraction occurs is called systole. In a blood pressure reading, the systolic pressure is typically the first number recorded. For example, with a blood pressure of 120/80 (120 over 80), the systolic pressure is 120. By 120 is meant 120 mm Hg (millimeters of mercury).